source: https://www.securityfocus.com/bid/17983/info

MonoChat is prone to an HTML-injection vulnerability because the application fails to properly sanitize user-supplied input before using it in dynamically generated content. 

Attacker-supplied HTML and script code would be executed in the context of the affected website, potentially allowing an attacker to steal cookie-based authentication credentials or to control how the site is rendered to the user; other attacks are also possible.

MonoChat 1.0 is reported to be affected. Other versions may be vulnerable as well.

Insert in your message :

<script>alert("Hacked by mdk")</script>

then go to monochat_affichage.php3